An integral aspect of all cancers is the requirement for cell division. Cytokinesis is the final step in cell division, where the cell constricts (usually at the equator) to partition the newly duplicated nuclei, along with half of the cytoplasm, into the resulting daughter cells. Many proteins that control cell division have been identified recently, yet those that control cytokinesis remain largely unknown. The long-term goals of the proposed research are to identify the molecules involved in cytokinesis and to understand how these molecules are involved in this process. The strict spatial and temporal requirements of cytokinesis suggests that it must be a very tightly regulated process. Small UTP-binding proteins have been implicated in the regulation of this process yet it is not known how. Recently, the eukaryotic organism Dictyostelium discoideum has emerged as one of the premier model systems for studying cytokinesis. Through a molecular- genetic screen using Dictyostelium we have discovered a novel small GTP- binding protein, named racE, that is absolutely required for cytokinesis. In order to understand how racE is involved in cytokinesis we need to know which domains of this protein are unique to racE and required for it to function in cytokinesis. By generating a number of chimeric proteins combining racE with another closely related small GTP- binding protein, that is not required for cytokinesis, we will identify the domain(s) of racE that are essential to its functioning in cytokinesis. This information will be used to isolate additional proteins that interact specifically with these domains. In this way we can begin to dissect the pathway through which racE is involved in cytokinesis. The genes coding for these racE-interacting proteins will be cloned and sequenced. In addition, their roles in cytokinesis will be investigated through genetic and biochemical means. The identification of the proteins required for cytokinesis, and how they interact with one another, will greatly increase our understanding of how this process is regulated in cells.